Organic compounds are brought into an excited state by the absorption of light. Through this excited state, various reactions (photochemical reactions) are caused in some cases, or luminescence is generated in some cases. Therefore, the organic compounds have a wide range of applications.
As one example of the photochemical reactions, a reaction of singlet oxygen with an unsaturated organic molecule (oxygen addition) is known. Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by direct photoexcitation. However, in the presence of another triplet excited molecule, singlet oxygen is generated to cause an oxygen addition reaction. In this case, a compound capable of forming the triplet excited molecule is referred to as a photosensitizer.
As described above, for generation of singlet oxygen, a photosensitizer capable of forming a triplet excited molecule by photoexcitation is needed. However, the ground state of an ordinary organic compound is a singlet state; therefore, photoexcitation to a triplet excited state is forbidden transition and generation of a triplet excited molecule is difficult. A compound that can easily cause intersystem crossing from the singlet excited state to the triplet excited state (or a compound that allows the forbidden transition of photoexcitation directly to the triplet excited state) is thus required as such a photosensitizer. In other words, such a compound can be used as the photosensitizer and is useful.
The above compound often exhibits phosphorescence. Phosphorescence refers to luminescence generated by transition between different energies in multiplicity. In an ordinary organic compound, phosphorescence refers to luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, fluorescence refers to luminescence in returning from the singlet excited state to the singlet ground state). Application fields of a compound capable of exhibiting phosphorescence, that is, a compound capable of converting the triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element including an organic compound as a light-emitting substance.
This light-emitting element has a simple structure in which a light-emitting layer including an organic compound that is a light-emitting substance is provided between electrodes. This light-emitting element has attracted attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. Further, a display device including this light-emitting element is superior in contrast, image quality, and wide viewing angle.
The light-emitting element including an organic compound as a light-emitting substance has a light emission mechanism that is of a carrier injection type: a voltage is applied between electrodes where a light-emitting layer is interposed, electrons and holes injected from the electrodes recombine to put the light-emitting substance into an excited state, and then light is emitted in returning from the excited state to the ground state. As in the case of photoexcitation described above, types of the excited state include a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio thereof in the light-emitting element is considered to be S*:T*=1:3.
At room temperature, a compound capable of converting a singlet excited state into luminescence (hereinafter, referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), not luminescence from the triplet excited state (phosphorescence). Accordingly, the internal quantum efficiency (the ratio of the number of generated photons to the number of injected carriers) of a light-emitting element including the fluorescent compound is thought to have a theoretical limit of 25%, on the basis of S*:T*=1:3.
On the other hand, in a case of a light-emitting element including the phosphorescent compound described above, the internal quantum efficiency thereof can be improved to 75% to 100% in theory; namely, the emission efficiency thereof can be 3 to 4 times as much as that of the light-emitting element including a fluorescent compound. Therefore, the light-emitting element including a phosphorescent compound has been actively developed in recent years in order to achieve a highly efficient light-emitting element. An organometallic complex that contains iridium or the like as a central metal is particularly attracting attention as a phosphorescent compound because of its high phosphorescence quantum yield (refer to Patent Document 1, Patent Document 2, and Patent Document 3).